various fixups for gdth

Cleans up various oddities found during a code audit, and drops the
legacy ISA support which hasn't had a chance to actually work for a
long time.

Follow-Ups:
- [PATCH 06/10] gdth: remove dead rtc code
  - From: Christoph Hellwig
- [PATCH 07/10] gdth: remove dead dma statistics code
  - From: Christoph Hellwig
- [PATCH 05/10] gdth: remove direct serial port access
  - From: Christoph Hellwig
- [PATCH 04/10] gdth: remove ISA and EISA support
  - From: Christoph Hellwig
- [PATCH 10/10] gdth: use generic DMA API
  - From: Christoph Hellwig
- [PATCH 09/10] gdth: remove interrupt coalescing support
  - From: Christoph Hellwig
- [PATCH 08/10] gdth: remove dead code under #ifdef GDTH_IOCTL_PROC
  - From: Christoph Hellwig
- [PATCH 03/10] gdth: remove gdth_{alloc,free}_ioctl
  - From: Christoph Hellwig
- [PATCH 02/10] gdth: reuse dma coherent allocation in gdth_show_info
  - From: Christoph Hellwig
- [PATCH 01/10] gdth: refactor ioc_general
  - From: Christoph Hellwig